The past decade has seen the development of a class of absorbents having the ability to absorb many times their own weight in water or other aqueous solutions such as blood, urine and other body exudates.
These materials are commonly referred to as superabsorbents or hydrogels. They are generally prepared by polymerizing one or more monomers which when homopolymerized form a water-soluble polymer. To render them water-insoluble while maintaining their water-absorption characteristics, the polymers are typically cross-linked, either covalently or ionically, to introduce a limited water-insolubility while retaining susceptibility to swelling in water and water-containing fluids.
Typically superabsorbents are subject to the phenomenon known as "gel blocking". This term, as discussed in more detail below, refers to the restriction of further liquid-induced swelling of the remaining dry portion of the superabsorbent by that portion of the superabsorbent that is first contacted by the aqueous fluid. The portion first contacted by the aqueous fluid becomes swollen or gelled and restricts access of the fluid to the remaining dry portion of the superabsorbent. The result is that only a portion of the superabsorbent functions effectively as an absorbent with concomitant decreased efficiency and increased costs.
U.S. Pat. No. 4,043,952 teaches a method of improving the aqueous dispersability or wet-out of a water-absorbent composition based on an anionic polyelectrolyte polymer by surface treating the polyelectrolyte to ionically complex the exposed surface of the absorbent composition. In the words of U.S. Pat. No. 4,043,952, "The ionic complexing of the surface is believed to retard the formation of a surface gel which inhibits the passage of an aqueous fluid into the interior of the absorbent composition." This surface treatment of the anionic polyelectrolyte does not completely solve the gel-blocking problem where a body of the superabsorbent anionic polyelectrolyte is to be contacted with aqueous fluid and does not allow use of the entire water-absorptive properties of the gel.
U.S. Pat. No. 4,090,013 to Ganslaw et al. discloses a water-swellable, water-insoluble absorbent. It is an ionic complex of a water-soluble anionic polyelectrolyte and a polyvalent metal cation. The composition is characterized by an ability to uncomplex at elevated pH and then again complex at lower pH. The material acts as a gel maintaining its integrity at low pH as it swells with absorbed water. At higher pH, above about 8.5, the material becomes soluble. This feature is utilized as an aid in formation in materials, which may be extruded as a high pH liquid but which become a cross-linked material when the pH is lowered after extrusion.
There remains a need for a hydrogel material that is not subject to gel blocking. There further remains a need for a swellable hydrogel material that has a delayed-swelling action that permits permeation of the material by water. Further there remains a need for a low-cost formation method for formation of hydrogel materials.